backfire

[Bill Saidel <saidel@crab.rutgers.edu>]

Here's another ignorant (albeint not unlearnable) tuning-beginner's question:
What does backfire mean?

The details are
Not under load, the car in neutral runs up to 4000-4500 purringly.
But take it out on the road and it backfires at 2,3,4000. So I haven't gone 
very far yet.

Could someone send me a simple lesson on the meaning of backfire?
TIA,
Bill,
76B with the 73 carbs